There's sometimes a bug in the firmwares of routers. I forget the specific setting name now but it has to do with the router disconnecting with no activity after x amount of time. "0" is supposed to mean never disconnect, but sometimes it did not work. One possible problem, loss of Internet. I suspected two possible issues as the culprit: the router has a bug and honors the 0 and never connects; ISP detects the 0, and doesn't allow connection. I like to look up whatever the ISP sets their provided routers and match.
To anyone curious/interested: AT&T fiber and Giggle Fiber (Cable Modem) default to 1 Day. Frontier fiber defaults to 1 hour...